Cubic boron nitride preparation utilizing nitrogen gas

ABSTRACT

Cubic boron nitride is deposited on a substrate by an activated reactive evaporation method involving heating a substrate in a vacuum, evaporating metal vapors into a zone between the substrate and the metals source, said source consisting of pure boron, or boron and a material selected from the group consisting of the elements chromium, nickel, cobalt, and manganese co-evaporated, or an alloy consisting essentially of from 0.1 weight percent to 5.0 weight percent of at least one of the elements chromium, nickel, cobalt, and manganese, the balance being boron, or an alloy consisting essentially of at least 60 percent by weight to the balance of boron with from 0.2 to 12 percent by weight of aluminum, and at least 0.2 to 24 percent by weight of at least one of cobalt, nickel, manganese, or other aluminide forming element; introducing nitrogen gas into the zone, and generating an electrical field in the zone in order to ionize the metal vapors and gas atoms in the zone with an electrically negative bias impressed on the substrate, the value of the bias depending upon the particular activated reactive evaporation process utilized.

BACKGROUND OF THE INVENTION

Present day commercially available boron nitride having a cubicstructure at room temperature generally has been prepared by using highpressure technology and processing techniques. Cubic boron nitride (CBN)produced by this process is expensive because of the complexity andattendant low volume associated with such high pressure processing.

Cutting tools coated with CBN are particularly attractive because highercutting rates, compared to tungsten carbide, are possible. Also becausea coolant can be used with CBN cutting tools, better surface finishesusually can be obtained than when using the Al₂ O₃ -TiC-TiN-TiO₂ typecutting tools dry.

U.S. Pat. No. 3,918,219, Wentorf, Jr., et al., discloses the conversionof CBN from hexagonal boron nitride (HBN) using certain aluminum alloysas catalysts. This process is carried on at a high pressure and hightemperature. CBN can be formed from BN at temperatures near 1800° C. andat pressures of approximately 85 kilobars with various materials such asalkali and alkaline earth metals, and aluminide forming materials ascatalysts.

U.S. Pat. No. 3,791,852, Bunshah, describes a process and apparatus forthe production of carbide films by physical vapor deposition by means ofan activated reactive evaporation deposition technique. This patent,including its drawing figure of a typical apparatus and descriptionthereof, is incorporated in this disclosure by reference.

U.S. Pat. No. 4,297,387, Beale, discloses a process of making CBNutilizing an activated reactive evaporation technique which involvesevaporating an alloy of boron and aluminum and at least one of cobalt,nickel, manganese, or other aluminide-forming elements in the presenceof ammonia gas while generating an electrical field in the zone forionizing the metal vapors and gas atoms in the zone. The temperature ismaintained at least at 300° C. with a preferred temperature of between500° C. and 1100° C. with the ammonia gas pressure preferably between1×10⁻⁴ torr to 8×10⁻³ torr.

U.S. Pat. No. 4,297,387, Beale, relies on the vaporization of a cubicphase nucleator which also functions as a barrier to possibledislocation motion or transformation nuclei formation in the form of analuminide made from cobalt, nickel, manganese, zirconium, or iron.

This patent also utilizes ammonia (NH₃) gas which is hygroscopic.Because it is hygroscopic, it presents difficulties with respect to theintroduction of impurities.

In applicant's copending patent application filed on the same date asthis patent application, there is disclosed a process involving theformation of CBN at relatively low temperatures utilizing the activatedreactive evaporation deposition process without the need to includealuminum as an alloying element in the metal alloy source so as to forman aluminide. However, this patent application utilizes ammonia gas as asource of nitrogen.

SUMMARY OF THE INVENTION

The present invention involves the formation of CBN at relatively lowtemperatures utilizing the activated reactive evaporation depositionprocess utilizing nitrogen gas rather than ammonia gas as a source ofnitrogen and thus permits the operation of the process at a preferredtemperature that is significantly below that which would otherwise bethe case and in fact below 500° C. The metals source may be pure boron,or boron and a material selected from the group consisting of chromium,nickel, cobalt, or manganese co-evaporated or an alloy comprising from0.1 weight percent to 5.0 weight percent of at least one of the elementschromium, nickel, cobalt, or manganese with the balance being boron, oran alloy of at least 60 percent by weight to the balance of boron withfrom 0.2 to 12 percent by weight of aluminum, and at least 0.2 to 24percent by weight of at least one of cobalt, nickel, manganese or otheraluminide-forming elements. In order to have the nitrogen gas functionto form cubic boron nitride, it is necessary to have a plasma presentduring the deposition process and an electrically negative bias must beimpressed on the substrate. The value of the required bias voltage is afunction of the particular coating deposition process utilized.

DETAILED DESCRIPTION OF THE INVENTION

The invention involves the deposition of CBN on a substrate throughactivated reactive evaporation including the steps of (a) heating thesubstrate in a vacuum, (b) evaporating metal vapors into a zone betweenthe substrate and the metals source, (c) introducing nitrogen gas intothe zone, and (d) generating an electrical field in the zone in order toionize the metal vapors and gas atoms in the zone in the presence of aplasma with an electrically negative bias impressed on the substrate,thus producing a deposit of CBN on the surface of the substrate.

The metal source may consist of pure boron, or an alloy consistingessentially of from 0.1 weight percent to 5.0 weight percent of at leastone of the elements chromium, nickel, cobalt, or manganese, the balancebeing boron, or an alloy consisting essentially of at least 60 percentby weight to the balance of boron with from 0.2 to 12 percent by weightof aluminum and at least 0.2 to 24 percent by weight of at least one ofcobalt, nickel, manganese, or other aluminide-forming element. The metalvapors may also be formed by co-evaporating boron and a materialselected from the group consisting of chromium, nickel, cobalt, ormanganese. It is important that a plasma be present during thedeposition process and that an electrically negative bias be impressedon the substrate. The value of the required electrically negative biasvoltage is a function of the particular metal vapor generating processused in accordance with the following:

    ______________________________________                                        Process             Bias Voltage                                              ______________________________________                                        Reactive Evaporation                                                                              -500 to -3000 volts                                       (Electron Beam)                                                               Reactive Sputtering (DC)                                                                          -200 to -3000 volts                                       Reactive Sputtering (RF)                                                                          -100 to -3000 volts                                       Reactive Sputtering (Magnetron)                                                                   -500 to -3000 volts                                       Arc Discharge       -20 to -2000 volts                                        ______________________________________                                    

The use of such bias voltage allows the deposition temperature to bereduced below 500° C.

To practice the invention's method, one utilizes a vacuum chamberapparatus, such as illustrated in the aforementioned U.S. Pat. No.3,791,852, Bunshah. Such an apparatus includes a vacuum chamber whichmay comprise a conventional cover or dome resting on a base with asealing gasket at the lower rim of the cover. A support and feed unitfor a source metal rod may be mounted in the base. The unit includes amechanism for moving the metal rod upward at a controlled rate. Coolingcoils may be mounted in the unit and supplied with cooling water from acooling water source. An electron gun is mounted in the unit andprovides an electron beam along the path to the upper surface of themetal rod, with the electron gun being energized from a power supply.

A substrate on which the CBN is to be deposited is supported in a frameon a rod projecting upward from the base. The substrate is heated by anelectric resistance heater supported on a bracket. Energy for the heateris provided from a power supply via a cable. The temperature of thesubstrate is maintained at a desired value by means of a thermocouple incontact with the upper surface of the substrate, with the thermocoupleconnected to a controller by a line, with the controller output signalregulating the power from the supply to the heater.

The desired low pressure is maintained within the vacuum chamber by avacuum pump connected to the interior of the chamber via a line.Nitrogen gas from a supply is introduced into the zone between the metalrod and substrate via a line and nozzle. A shutter is mounted on a rodwhich is manually rotatable to move the shutter into and out of positionbetween the metal rod and substrate.

A deflection electrode, typically a tungsten rod, is supported from thebase in the reaction zone between the metal rod and substrate. If themetal vapor source is a sputtering cathode, no deflection electrode isnecessary. An electric potential is provided for the rod from a voltagesupply via a line. An electric insulating sleeve, typically of glass, isprovided for the rod within the vacuum chamber, with the metal surfaceof the rod exposed only in the zone between the source and substrate.When a potential is connected to this electrode, some of the electronsfrom a region just above and/or adjacent to the molten pool, liberatedat the end of the rod, are attracted to the reaction zone. The pool isthe preferred source of electrons for the electrode, but a separateelectron emitter could be added if desired.

Various components utilized in the apparatus described above may beconventional. The evaporation chamber may be a 24 inch diameter and 36inch high water cooled stainless steel bell jar. The vacuum pump may bea 10 inch diameter fractionating diffusion pump, with anantimigration-type liquid nitrogen trap. The source metal unit may be aone inch diameter rod fed electron beam gun, self-accelerated 270°deflection type, such as Airco Temescal Model R1H-270. The power supplymay be an Airco Temescal Model CV30, 30 kw unit which may be operated ata constant voltage such as 10 kilovolts, with a variable emissioncurrent.

Various sizes and shapes of substrates can be utilized. A typicalsubstrate is a three inch by ten inch metal sheet in the order of fivemils thick. Various metals have been used including stainless steel,titanium, and zirconium. Other substrate materials can be used includingtool steels and carbides, such as WC/C alloy and SiC. In one embodimentthe substrate is based about eight inches above the surface of the metalsource. The heater may be an 18 kilowatt tantalum resistance heaterproviding for heating the substrate to 700° C. and higher temperatures,as desired.

The metals source material may be a solid rod or billet and for thejust-described feed unit may approximate about one inch in diameter andsix inches in length. The solid rod for the metals source may beprovided by a preparation method in which particulate metals mixedtogether in the amount providing the desired requisite composition aremelted and molten drops therefrom dripped or flowed into a water-cooledcylindrical mold of appropriate material and size.

Either pure boron or an alloy consisting essentially of 0.1 weightpercent to 5.0 weight percent of at least one of the elements chromium,nickel, cobalt, or manganese, the balance being boron may be used as themetals source. If desired, an alloy consisting of at least 60 percent byweight to the balance of boron with from two to 12 percent by weight ofaluminum, and at least 0.2 to 24 percent by weight of at least one ofcobalt, nickel, manganese or other aluminide-forming element may beutilized as a metals source. If desired, dual source processing may beused to reduce the cost of fabrication and also ease the degree ofcontrol necessary to accomplish the formation of CBN. In this case, morethan one source metal rod is utilized. In such a case, one rod would bepure boron and the other rod would be chromium, nickel, cobalt, ormanganese.

The present invention employs nitrogen gas as the source of nitrogen information of the CBN placed on the substrate. This nitrogen gas isadmitted or introduced generally through needle valves at a rateadequately correlated with the rate at which the metal vapors areprovided so that preferably at least the stoichiometric amount ofnitrogen atoms from the nitrogen gas are present in the zone whereactivation or ionization of the gas atoms is provided. Of course, thepartial pressure of the introduced nitrogen gas needs be such thationization can be provided, such as by the usual electrical field. Apreferred range for gas pressure is about 1×10⁻⁴ torr to 8×10⁻³ torr.

Plasma activation is essential to effect the completion of the chemicalreaction to produce CBN.

It is also essential that an electrically negative bias be impressed onthe substrate. The value of the required bias voltage is a function ofthe particular coating deposition process employed as has been set forthabove and this permits the reduction of the required depositiontemperature to below 500° C.

Although the invention has been described as applied to sheetsubstrates, other configurations are within the scope of the inventionsuch as tool shapes which may have CBN deposited thereon. For example:reamers, gear cutters, and the like, may have CBN deposited thereonsince there are very few restrictions on substrate geometry that existin evaporating and coating.

Additionally, it is within the realm of the invention to contemplateproducing a coating deposit-substrate interdiffusion, e.g., by heattreatment, to improve deposit adherence. Further, deposition of variousalloying elements along with the CBN is contemplated for variouspurposes, such as improving deposit ductility. It has been noted thatslight amounts of nickel deposited along with the CBN increases depositductibility.

While this invention has been described in its preferred embodiment, itis to be appreciated variations therefrom may be made without departingfrom the true scope and spirit of the invention.

What is claimed is:
 1. A method for depositing cubic boron nitride on asubstrate, comprising the steps of:(a) heating a supported substrate ina vacuum; (b) supplying metal vapors into a zone between said substrateand a metals source, said source comprising a material selected from thebroup consisting of:boron, boron and a material selected from the groupconsisting of the elements chromium, nickel, cobalt, and manganese, ametal alloy consisting essentially of from 0.1 weight percent to 5.0weight percent of at least one of the elements chromium, nickel, cobalt,and manganese, the balance being boron, and a metal alloy consistingessentially of at least 60 percent by weight to the balance of boronwith from 0.2 to 12 percent by weight of aluminum, and at least 0.2 to24 percent by weight of at least one of cobalt, nickel, manganese, orother aluminide-forming element; (c) introducing nitrogen gas into saidzone; (d) generating an electrical field in said zone ionizing the metalvapors and gas atoms in the zone; and (e) maintaining an electricallynegative bias impressed on said substrate depending upon the processemployed and in accordance with the following schedule:

    ______________________________________                                        Process             Bias Voltage                                              ______________________________________                                        Reactive Evaporation                                                                              -500 to -3000 volts                                       (Electron Beam)                                                               Reactive Sputtering (DC)                                                                          -200 to -3000 volts                                       Reactive Sputtering (RF)                                                                          -100 to -3000 volts                                       Reactive Sputtering (Magnetron)                                                                   -500 to -3000 volts                                       Arc Discharge       -20 to -2000 volts                                        ______________________________________                                    

whereby cubic boron nitride is deposited on the surface of saidsubstrate.
 2. The method of claim 1 including directing an electron beamonto the metals alloy source for providing the metal vapors byevaporation.
 3. The method of claim 1 wherein the metal vapors aresupplied by means of a sputtering cathode.
 4. The method of claim 1wherein the metal vapors are supplied by means of an arc discharge-typeevaporation or sublimation process.
 5. The method of claim 1 wherein thesubstrate is heated to a temperature below 500° C.
 6. The method ofclaim 5 wherein the nitrogen gas is introduced in an amount providing agas partial pressure between about 1×10⁻⁴ torr to 8×10⁻³ torr in saidzone.
 7. The method of claim 4 wherein the arc discharge-typeevaporation or sublimation process involves the use of a hot hollowcathode.